The process according to the invention provides, with few reaction steps, a novel, advantageous route to .alpha.-substituted ring systems. These ring systems include the tetrahydroanilines. These are interesting precursors of fine chemicals or crop protection agents, since further transformations at the double bond are possible. They are furthermore a possible source for cyclohexylamine which is used as a vulcanization promoter or corrosion inhibitor and as a precursor for synthetic sweeteners (for example for cyclamate).
According to the prior art that has hitherto been disclosed, the preparation of tetrahydroanilines requires a large number of steps or partial steps: P. Kocovsky (Synlett 1990, 677) describes a route to tetrahydroaniline derivatives by reacting cyclohexenone with hydroxylamine in pyridine to give cyclohexenone oxime which is subsequently acylated in pyridine and then reduced with lithium aluminium hydride. Using this process, it is possible to obtain good yields; however, a reduction with lithium aluminium hydride is not suitable for industrial application or the preparation of large amounts.
Y. Yang, F. Diederich and J. S. Valentine (J. Am. Chem. Soc. 1991, 113, 7195 and J. Am. Chem. Soc, 1990, 112, 7826) describe the oxidation of cyclohexene with iodosylbenzene in the presence of aluminium triflate and acetonitrile in yields of at most 13%. T. Shono and A. Ikeda (J. Am. Chem. Soc. 1972, 94, 7892) disclose the electrooxidation of cyclohexene in acetonitrile with small amounts of water; however, they obtain the desired tetrahydroaniline derivative only as by-product. They do not state the yield. T. Toshimitsu, H. Owada, T. Aoai, S. Uemura and M. Okana (J. C. S, Chem. Commun. 1981, 546) carry out a hydrogen peroxide oxidation of .alpha.-selenylphenyl-acetamidocyclohexane and obtain, at temperatures of 250.degree. C., the desired tetrahydroaniline derivative in very good yields, but with the aid of selenium, which is difficult to use industrially.
R. Jumnah, M. J. Williams, A. C. Williams (Tetrahedron Letters 1993, 34, 6619) and J. F. Bower, R. Jumnah, A. C. Williams, J. M. J. Williams (J. Chem. Soc., Perkin Trans. 1 1997, 1411) describe a process for preparing a tosylated tetrahydroaniline derivative by reacting, with palladium catalysis, O-acyl-2-cyclohexenol with tosylamide. Also described is a route starting from O-acetyl-2-cyclohexenol via palladium-catalysed reaction with an azide ion to give azido-2-cyclohexene and subsequent transformation of the azide with thioacetic acid to give acetamido-2-cyclohexene in an overall yield of 46%. These processes involve a high number of steps, and they sometimes use starting materials (azide) which are difficult to handle.
Y. Ichikawa, M. Yamazaki, M. Isobe (J. Chem. Soc. Perkin Trans. 1, 1993, 2429) and Y. Ichikawa, M. Osuda, I. I. Ohtani, M. Isobe (J. Chem. Soc., Perkin Trans. 1 1997, 1449) describe a process where initially 2-cyclohexenol is reacted with trichloroacetyl isocyanate and potassium carbonate to give O-carboxyamide-2-cyclohexenol which, in a subsequent step, is dehydrated to give O-cyano2-cyclohexenol which in turn is rearranged to give 2-cyclohexenyl isocyanate in good yields. This 2-cyclohexenyl isocyanate is then rearranged with trimethylaluminium to give the desired tetrahydroaniline derivative. It is particularly disadvantageous here that stoichiometric amounts of trichloroacetyl isocyanate and trimethylaluminium, which is difficult to handle in this rearrangement, are required.
C. Briguet, C. Freppel, J.-C. Richer and M. Zador (Can. J. Chem. 1974, 52, 3201) describe a process for the oxidation of cyclohexene with cerium ammonium nitrate in acetonitrile which contains 1% of water, and they obtain acetamido-2-cyclohexene. Here, the use of stoichiometric amounts of cerium ammonium nitrate stands in the way of industrial use. Y. Leblanc, R. Zamboni, M. A. Bernstein (J. Org. Chem. 1991, 56, 1971) describe an ene reaction of cyclohexene with bis-(2,2,2-trichloroethyl) azodicarboxylate at 155.degree. C. which, after work-up, affords a yield of 70% of the desired tetrahydroaniline derivative.
G. Kresze and H. Munsterer (J. Org. Chem. 1983, 48 3561) describe an ene reaction of cyclohexene with bis(methoxycarbonyl)sulphurdiimide to give a product which, after basic work-up, affords a tetrahydroaniline derivative. In the last two cases, the fact that the reagent has to be used in stoichiometric amounts stands in the way of commercial utilization.
JP-A-01 261 354 (Sumitomo Corp.) describes the best industrial process for the synthesis of tetrahydroaniline derivatives that has hitherto been disclosed. In an autoclave, 1,2-dichlorohexane is reacted in the presence of ammonia in isopropanol to give 2-cyclohexeneamine in a yield of 80%. However, the fact that it is necessary to obtain 1,2-dichlorohexane first, and the high expense in apparatus for carrying out autoclave reactions make this process appear to be not particularly advantageous.
There was therefore a need for a simpler route to tetrahydroaniline, providing a considerable relief for the environment and simultaneously reducing production costs.